Add the AMD MONITORX/MWAITX feature definition introduced in
Bulldozer/Ryzen CPUs.

Hide the AMD MONITORX/MWAITX capability.
Otherwise, recent Linux guests will use these instructions, resulting
in #UD exceptions since bhyve doesn't implement MONITOR/MWAIT exits.

This fixes boot-time hangs in recent Linux guests on Ryzen CPUs
(and probably Bulldozer aka AMD FX as well).